One class per file. Each file exports the class, and doesn't set SIP.*
Let modules require() their own dependencies.

- LoggerFactory.js require()s Logger.js
- Session.js, Invite*Context.js require() Session/Constants.js
- Session.js require()s Session/DTMF.js
- WebRTC.js require()s WebRTC/Media{Handler,StreamManager}.js
- Incoming{Request,Response}.js require() IncomingMessage.js
- Dialogs.js require()s Dialog/RequestSender.js